OLEDs, as described by Tang in commonly assigned U.S. Pat. No. 4,356,429, are commercially attractive because they offer the promise of low cost fabrication of high density pixel displays exhibiting bright electroluminance (EL) with long lifetime, high luminous efficiency, low drive voltage, and wide color range.
A typical OLED includes two electrodes and one organic EL unit disposed between the two electrodes. The organic EL unit commonly includes an organic hole-transporting layer (HTL), organic light-emitting layer (LEL), and an organic electron-transporting layer (ETL). One of the electrodes is the anode, which is capable of injecting positive charges (holes) into the HTL of the EL unit, and the other electrode is the cathode, which is capable of injecting negative charges (electrons) into the ETL of the EL unit. When the anode is biased with a certain positive electrical potential relative to the cathode, holes injected from the anode and electrons injected from the cathode can recombine and emit light from the LEL. At least one of the electrodes is optically transmissive, and the emitted light can be seen through the transmissive electrode.
Significant efforts have been made in selecting suitable materials and forming different layer structures in OLEDs to achieve improved EL performance. Numerous OLEDs with alternative layer structures have been disclosed. For example, in addition to the three layer OLEDs that contain a LEL between the HTL and the ETL (denoted as HTL/LEL/ETL), there are other multilayer OLEDs that contain additional functional layers in the EL unit, such as a hole-injecting layer (HIL), an electron-injecting layer (EIL), an electron-blocking layer (EBL), or a hole-blocking layer (HBL), or the combination thereof. These new layer structures with new materials have indeed resulted in improved device performance.
As is known, electron transport in OLED is generally less efficient than hole transport. As a result, the electron-hole recombination in LEL is sometimes unbalanced with insufficient electrons. In order to achieve more efficient electron-hole recombination, much attention has been paid in forming an effective ETL or EIL using suitable electron-transporting material (ETM) in OLEDs. For example, the commonly used ETL or EIL in OLEDs includes the ETM of tris(8-hydroxyquinoline)aluminum (Alq), 4,7-diphenyl-1,10-phenanthroline (Bphen), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and 2,2′-[1,1′-biphenyl]-4,4′-diylbis[4,6-(p-tolyl)-1,3,5-triazine] (TRAZ).
According to prior art, the selection of practically useful organic ETMs in OLEDs is limited to those having a reduction potential less than −1.0 V vs. a Saturated Calomel Electrode (SCE) or having a LUMO less than 3.3 eV below the vacuum energy level (Evac). This is because the LEL in an OLED typically has a reduction potential less than −1.0 V vs. SCE. Therefore if any organic material having a reduction potential greater than −1.0 V vs. SCE were used as an ETL in the OLED, it would produce an electron injection barrier between the ETL and the LEL. Especially, if any organic material having a reduction potential greater than −0.5 V vs. SCE were used as an ETL in the OLED, it would produce an electron injection barrier greater than 0.5 eV. Electron injection from the ETL into the LEL would be very difficult or impossible, resulting in little or no EL emission from the OLED. However, the commonly used ETMs, although forming a low or no barrier for electron injection at LEL/ETL interface, exhibit a relatively high barrier for electron injection at the interface between the ETL and the cathode, especially when the cathode material has a work function higher than 4.0 eV. This high barrier interface between the ETL and the cathode can be modified by inserting a thin insulating layer or by doping the ETL with a material having a work function lower than 3.0 eV, but this interface will be degraded eventually during operation.
Therefore, it would be advantageous to find a way to broaden the material selection for ETL (EIL), to use the material having a reduction potential greater than that of commonly used ETMs, and to produce a stable interface between organic layer and the cathode in OLEDs.